Current sources of this type are generally used having small ratios G (up to about 10) of output current to input current. For these uses the second transistor has an emitter surface G times greater than that of the first transistor (or it is constituted by G individual transistors which are identical to the first transistor and arranged in parallel) so as to obtain the same base/emitter voltage drop in the first and second transistors, and avoid variations of the ratio G as a function of temperature.
For higher ratios G, for example, ranging to 100, such a solution leads to prohibitive dimensions for the second transistor, and in that case arrangements with an operational amplifier will be used. Such solutions are used, for example, by MATRA COMMUNICATION (French patent application 88 01645, dated Feb. 11, 1988, more particularly, FIG. 5), SGS-THOMSON (report of the TEA 7063 circuit--Telephone Speech and Peripherals Line Control) and MOTOROLA (Product preview of the TCA 3385 circuit--Telephone Ring Signal Converter).
These embodiments have the disadvantage of requiring the presence of an operational amplifier which takes up a relatively large space in the integrated circuit, and which, furthermore, may present problems of stability, especially if the circuit forms part of a complex arrangement presenting cascaded stages.